248 SCIENCE AND THE HUMAN MIND 



ditions. Characters may be linked in pairs so that 

 one cannot appear without the other, or again they 

 may be incompatible and never be present together. 



Many Mendelian characters have now been traced 

 in plants and animals ; while, as a practical guide 

 in breeding, the method has been successfully applied 

 to unite certain desirable qualities and to exclude 

 others of harmful tendency. By this means, Biffen 

 has established new and valuable species of wheat, 

 in which immunity to rust, high cropping power and 

 certain baking qualities have been brought together 

 in one and the same species owing to a long series 

 of experiments based on the Mendelian laws of 

 inheritance. 



The extension of this new method of research to 

 mankind at once cleared away many old puzzles and 

 opened up fresh fields of study. Many diseases and 

 malformations have been proved to be dominant 

 Mendelian characters ; while deaf-mutism, and other 

 defects, appear to be recessive, and therefore especially 

 liable to appear in cousin marriages between sound 

 individuals who come from families liable to the 

 affliction. 



Of normal healthy characters, eye colour alone has 

 yet definitely been proved to show Mendelian pheno- 

 mena in its line of descent, brown colour being domin- 

 ant to gray, though indications are not wanting 

 that further analysis will disclose similar relations in 

 many other directions. 



The application of such results is obvious. We can 

 already predict accurately the probable results of 

 human matings when the potential parents possess 

 a character which has proved to be Mendelian in 



